A new report reveals that mustard gas was used during fighting between Islamic State (IS, previously ISIS/ISIL) and other militant groups in a Syrian town, Reuters has reported, citing an undisclosed paper by the international chemical arms watchdog.

The news agency also quotes diplomatic sources as confirming the use of chemical weapons during the fighting between Islamic State and a rebel group in the town of Marea at the time.

“It raises the major question of where the sulfur mustard came from,” a source stated. “Either they gained the ability to make it themselves, or it may have come from an undeclared stockpile overtaken by IS. Both are worrying options.”

Syria surrendered its chemical weapons stockpile 18 months ago, which would have included sulfur mustard. It also officially became an OPCW member in October 2013. The move followed atrocities involving chemical weapons, such as an attack in Ghouta in August of 2013, which Damascus and anti-Assad rebels blamed on each other.

In August, Syria’s envoy to the UN categorically denied claims that the country had used chemical weapons. “The Syrian government has not used and will never use chemical weapons,” said Bashar Jaafari, who is the Syrian Ambassador to the UN. He also added that the Syrian army had been attacked by chemical weapons.

State Department spokesman John Kirby told Reuters that the OPCW shared three reports with member states on October 29.

He stressed that “two of these reports are very disturbing. The OPCW has confirmed [chemical weapons] use in one investigation, and concluded likely use in a second.”

The spokesman also revealed that OPCW had discovered the use of toxic chemicals, which likely contained chlorine, in opposition-controlled territory in Syria’s Idlib Province.

UN Secretary General Ban Ki-moon will get a chance to review all three reports later this month, according to Kirby. “If it is determined that a non-state actor such as ISIL was also using chemical weapons, such use would be consistent with ISIL’s record of complete disregard for human rights and international norms and values,” he said.

There were also reports that Islamic State fighters used mortar rounds containing a mustard agent against Kurdish peshmerga fighters in northern Iraq back in August, according to Kurdish authorities.

The Russian Foreign Ministry’s spokesperson, Maria Zakharova, warned in September that Islamic State had obtained the scientific documentation necessary to produce chemical weapons.

“In particular, there’s information about ISIS getting scientific and technical documentation for production of chemical weapons, capture of chemical plants, recruitment of foreign experts for activities aiming to set up synthesis of chemical warfare agents,” Zakharova told a media briefing.

The spokesperson added that there was a possibility that these chemical agents could be spread outside of the territory controlled by ISIS and even beyond the Middle East.